The present invention relates to an image reading apparatus which obtains image data from a reading object.
An image scanner obtains image data by opto-electrically converting a reflected light obtained by illuminating a sheet-like document, a book or a three-dimensional reading object, by an image reading sensor.
An image scanner is a transparent glass plate, for example, and includes a document holder (a document table) to hold a reading object placed on a document table, an illumination lamp, an illumination unit to illuminate a reading object placed on a document table, a line CCD sensor or an image reading sensor, and an image forming lens provided between a document table and a CCD sensor, to apply a reflected light from a reading object to a CCD sensor. An image forming lens is a reduction lens to reduce a reflected light of a reading width (length) defined as the length of one side of a document table, to meet the width (length) of the light receiving surface of a CCD sensor. In many cases, in an image scanner, by moving an illumination unit along a reading object fixed on a document table, a reflected light from a reading object is applied to the light receiving surface of a CCD sensor. Thus, an illumination lamp is made tubular long enough to illuminate the width of a reading object.
However, when a reduction lens is used, the intensities of the light passing through the center and periphery of a lens are not constant, and it is known that the light intensity lowers as the light shifts from the lens center, according to the well-known cosine biquadratic law.
Therefore, when a xenon lamp is used as an exposing lamp, for example, the shape of an external electrode is characterized to increase the light intensity in the area shifted away from the lens center. However, even if a xenon lamp with the external electrode shaped as above is used, it is difficult to make the illuminance uniform on the light receiving surface of a CCD sensor. Further, though it is proposed to compensate the influence of the cosine biquadratic law between the lights passing through the center and periphery of a lens when making shading compensation of the opto-electrically converted output from a CCD sensor, it has been know that the S (Signal)/N (Noise) ratio lowers in the light passing through the lens periphery.
Moreover, the characterizing the external electrode shape of a xenon lamp increases the cost of the lamp.